Plant Cytogenetics: Inheritance of Lethal genes.
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The document discusses the inheritance of lethal genes, which deviate from Mendelian genetics by reducing viability in individuals or causing death. It provides examples including chlorophyll content in snapdragons and coat color in mice, highlighting altered genotypic and phenotypic ratios in offspring. The document also covers human diseases linked to recessive lethal alleles, such as cystic fibrosis and achondroplasia.
Plant Cytogenetics: Inheritance of Lethal genes.
- 1. B.SC. III: SEMESTER-VI PAPER XIX: GENETICS AND BIOTECHNOLOGY INHERITANCE OF LETHAL GENES Dr. A.S. Naik, Head, Dept. of Botany SBES College of Science
- 2. Explanation of inheritance of lethal genes. This is a deviation from Mendel’s classic onohybrid inheritance. ◦ Definition: Genes which result in viability reduction of individual or become cause of death of individuals that carry them. ◦ F2 shows genotypic ratio other than Mendel’s monohybrid ratio, as 2:1 and two phenotypic classes also in same proportion. ◦ Some examples: 1. Genes controlling chlorophyll content in leaves. 2. Coat colour in mice: Yellow, black, agouti 3. Hutington disease in humans.
- 4. Types as per time of expression.
- 7. Inheritance of lethal genes in Antirrhinum majus (Snapdragon) ◦ E. Baur (1907) observed lethal gene in Snapdragon (Antirrhinum) and found that it is characterized by variegated leaves. ◦ The “golden” variety on selfing gives rise to 2 types of offspring, golden and green in the ratio of 2:1 instead of 3: 1. ◦ The golden ones are heterozygous and the green ones breed true being recessive homozygous. ◦ The recessive allelic combination results in plants with no chlorophyll (albino) that die.
- 9. Inheritance of lethal genes in Antirrhinum • Genotype GG: Dark green leaves with adequate chlorophyll ◦ Gg: golden-green leaves (auria) ◦ Genotype-gg: White (colourless) leaves. ◦ Genotype GG breeds true-only green leaved plants ◦ If parents with aurea leaves crossed: Gg X Gg ◦ gametes of Gg: G and g ◦ Will produce F2 with changed genotypic and phenotypic ratios.
- 10. Male parent Female parent G g G GG (Green) Gg (Aurea) g Gg (Aurea) gg (albino, Lethal) Genotypic Ratio of F2: GG ¼: Gg 2/4: gg ¼ (Lethal, dies) Hence gets modified as 2:1 Phenotypic ratio: Aurea : Green-2:1 F2:
- 11. Other examples: Coat colour in mice. ◦ Example of dominant lethal genes ◦ Dominant allele in homozygous condition proves lethal.
- 12. Lethal genes in humans. ◦ Examples of human diseases caused by recessive lethal alleles include cystic fibrosis, sickle-cell anemia, and achondroplasia. Achondroplasia is an autosomal dominant bone disorder that causes dwarfism. While the inheritance of one achondroplasia allele can cause the disease, the inheritance of two recessive lethal alleles is fatal. ◦
- 13. Assignment on Lethal genes 1. In Snapdragon, Green plants are produced by gene ‘C’ that controls adequate production of chlorophyll. It is dominant over its allele ‘c’, that produces no chlorophyll and plants are albino. The homozygous condition of the ‘c’ is lethal due to total lack of chlorophyll whereas heterozygous condition ‘Cc’ produces yellow-green/Auria plants. ◦ What genotypic and phenotypic proportion can be expected from the following crosses? (a) CC X Cc (b) Cc X Cc (c) CC X cc (d) Cc X cc ◦ 2. In cattle, gene ‘A’ produces phenotypically normal legged animals and is dominant over its allele ‘a’ producing animals with amputated legs. The gene ‘A” in homozygous recessive condition-‘aa’ has a lethal effect on animals. Find out phenotypic proportion of the offspring expected from the following crosses. ◦ (a) AA X Aa (b) Aa X Aa (c) AA X aa (d) Aa X aa